1. Field of the Invention
This invention relates to a control system and method for continuously preparing mixtures of powdered solids in liquids, removal of entrained bubbles from such mixtures, and for controlling such processes. The invention is particularly useful for the preparation of bubble-free slurries of finely ground polyurethane-foam particles in polyol, and for the preparation of new polyurethane articles that contain the finely ground polyurethane-foam particles.
2. Background of the System
“Polyurethane” (PUR) describes a general class of polymers prepared by polyaddition polymerization of polyisocyanate molecules and one or more active-hydrogen compounds. “Active-hydrogen compounds” include water and polyfunctional hydroxyl-containing (or “polyhydroxyl”) compounds such as diols, polyester polyols, and polyether polyols. Active-hydrogen compounds also include polyfunctional amino-group-containing compounds such as polyamines and diamines. A common example of an active-hydrogen compound is a polyether polyol such as a glycerin-initiated polymer of ethylene oxide or propylene oxide. Polyether polyols useful for slabstock flexible polyurethane foams generally have a molecular weight in the range of 2000 to 6000 g/mol, a functionality in the range of 2 to 7 (preferably about 3), and a viscosity at 25° C. generally in the range of 100 to 10,000 mPa-s.
“PUR foams” are formed via a reaction between one or more active-hydrogen compounds and a polyfunctional isocyanate component, resulting in urethane linkages. PUR foams are widely used in a variety of products and applications. These foams may be formed in wide range of densities and may be flexible, semi-flexible, semi-rigid, or rigid foam structures. Generally speaking, “flexible foams” are those that recover their shape after deformation. In addition to being reversibly deformable, flexible foams tend to have limited resistance to applied load and tend to have mostly open cells. “Rigid foams” are those that generally retain the deformed shape without significant recovery after deformation. Rigid foams tend to have mostly closed cells. “Semi-rigid” or “semi-flexible” foams are those that can be deformed, but may recover their original shape slowly, perhaps incompletely. A foam structure is formed by use of so-called “blowing agents.” Blowing agents are introduced during foam formation through the volatilization of low-boiling liquids or through the formation of gas due to chemical reaction. For example, a reaction between water and isocyanate forms carbon dioxide (CO2) gas bubbles in PUR foam. This reaction also generates heat and results in urea linkages in the polymer. Additionally, surfactants may be used to stabilize the polymer foam structure during polymerization. Catalysts are used to initiate the polymerization reactions forming the urethane linkages and to control the blowing reaction for forming gas. The balance of these two reactions, which is controlled by the types and amounts of catalysts, is also a function of the reaction temperature. A typical foam recipe includes at least one polyol, at least one isocyanate, and also typically includes water, surfactant, and catalysts, and also optionally includes additional blowing agent, fillers, and additives for color, fire performance, antimicrobial activity, etc.
Polyurethane foam can be ground into fine particles using, for example, cryogenic processes or roll mills (see for example, U.S. Pat. No. 6,670,404, incorporated herein by reference). These fine particles can then be used to replace chemicals in recipes for new foam. The use of recycled polyurethane foam provides an environmental benefit and often a cost savings.
In order to add polyurethane powder to the recipe, the powder must be mixed with liquid reactants. Further, the slurry must be free of entrained bubbles because they create an undesirable irregular cell structure in the foam, including holes and splits. Powder can be mixed with liquid reactants in a batch process by adding a known mass of powder to a known mass of polyol, mixing thoroughly, and allowing sufficient time (generally about 8 to 48 hours) for entrained bubbles to leave the slurry. Such a natural de-gassing process takes a long time because the slurry has a high viscosity, generally about 500 to 20,000 mPa-s. A continuous process for removal of entrained air is preferred over a batch process because the continuous process would not require waiting for entrained air to leave the slurry naturally, and the continuous processes would not require the large storage capacity necessary to hold the slurry needed for an entire day of foam production.
Methods are desirable for producing bubble-free slurries of fine particles of polyurethane in polyol, and for producing foam with these slurries. Particularly, a need exists for devices and processing methods to take finely ground polyurethane-foam particles, disperse them as a slurry in polyol, remove substantially all entrained bubbles from the slurry, and to use this slurry as a direct replacement for at least some of the polyol in the production of new polyurethane articles. It is further desirable for such a process to be able to run continuously, such that powder and polyol are mixed, de-gassed, and used to make new polyurethane articles without delay. The desired continuous process must be able to deliver bubble-free slurry with an accurately controlled solids concentration at an accurately controlled flow rate.